The present invention relates to the field of digital modulation of sinusoidal carriers, and more specifically to a process for actuation of multi-level digital modulation by a digital signal processor.
In multi-level digital modulation, the modulating signal is generally in the form of a flow of serial bits with a frequency of fb bits. This flow is converted into N parallel flows of bits (N=1, 2, 3 . . . ) of which the N bits, which are present simultaneously in the N flows, form words denominated by symbols having a symbol frequency fs=fb/N.
Each N bit symbol can express a number of 2N different combinations of bits. The number 2N is termed the modulation level.
For low modulation levels (2N=2, 4, 8), there is ordinarily used PSK (Phase Shift Keying) modulation which associates with each symbol one phase of a carrier.
For higher levels of modulation, recourse is usually had to QAM (Quadrature Amplitude Modulation) modulation which associates with each symbol not only the phase, but also the level, of a carrier. The possible 2N values of the phases and level combinations of the modulated carrier are generally represented by a constellation of points in a Cartesian plane, the axes of which represent two mutually sinusoidal quadrature carriers.
Each point of the constellation is identified by a vector which departs from the origin of the plane. The components of the vectors in relation to the Cartesian axes are obtained directly from the symbols by an operation termed `mapping` which associates with each symbol two other symbols whose values are the above components. The associated symbols form two flows with frequency fs, termed `in phase` channel I and `in quadrature` channel Q, respectively.
In a conventional QAM modulator, the symbols of the I and Q channels are converted from digital to analog and are filtered with two shaping filters to appropriately shape the spectrum of the two analog signals obtained. These signals are then used to modulate two synchronous sinusoidal carriers in quadrature with each other. The modulated carriers are added together to obtain a single modulated carrier in the desired QAM mode.
Shaping of the above mentioned spectrum is performed by a filtration described as `optimum` for the symbols belonging to the in phase and in quadrature channels.
In view of the foregoing, a conventional QAM modulator includes the following:
a series/parallel converter to convert the serial input flow into N parallel bit flows; PA1 a mapping memory to obtain the I and Q channels starting from the N parallel flows; PA1 two digital/analog converters for conversion of the symbols of the I and Q channels into continuous values; PA1 two `optimum` analog filters placed after said converters; PA1 two analog multipliers to whose first inputs arrive the output signals from the `optimum` filters, to whose second inputs arrive two sinusoidal in quadrature carriers, and whose outputs are the above said amplitude modulated carriers respectively, and PA1 an analog adder to whose inputs arrive the outputs of the multipliers and whose output is a single QAM modulated carrier.
The conventional modulator, however, has a serious drawback due to the fact that the gain of the analog multipliers shows strong tolerances and is susceptible to thermal drift which introduces phase and amplitude inaccuracies in the modulated signal. The consequences of these inaccuracies are noticed mainly at the higher modulation levels (N&gt;4).
These shortcomings are overcome by having recourse to QAM modulators of a second type provided in a known manner completely in the digital mode.
Such modulators do not require the two digital/analog converters in the I and Q channels as in the above converters because the respective symbols undergo the `optimum` filtration directly in the digital mode. The filtered symbols are also multiplied digitally by the values of the sinusoidal in quadrature carriers appropriately digitalized. The digital samples of the product are converted into analog and are filtered by means of a low pass filter, termed `reconstruction`, to eliminate the unwanted spectral components and to obtain the modulated sinusoidal carrier QAM.
As is known, to digitally filter signals, it is first necessary to sample them with a sampling frequency fc whose value must be equal to at least twice the maximum frequency contained in the band of the signal to be sampled. In the case in question, the signals to be sampled correspond to the symbols of the I and Q channels, and the maximum frequency corresponds to the symbol frequency fs.
The spectrum of a sampled signal is formed of an infinite series of spectra of the signal in base band placed around whole multiples of the frequency fc constituting overall a repetition spectrum.
To further space the repeated spectra, it is useful to perform an oversampling of the symbols at the frequency fc=fs .times.K where K is a whole number&gt;2 representing the number of samples per symbol.
The value of K is selected so that the distance between two repeated spectra is broad enough for an embodiment of the reconstruction filter with a slope which is not overly steep for the attenuation characteristic with the frequency.
The QAM modulators of the second type have, however, a considerable circuit complexity due mainly to the high number of multipliers included in the digital filter placed on the I and Q channels, a number which will be greater in proportion to the accuracy of the filter and the higher the sampling frequency fc selected.